A power storage system using a sodium-sulfur battery includes the required number of module batteries that are combined depending on the storage capacity. Each module has a large number such as hundreds of cells contained inside a heat-insulating container. The module batteries are housed in multiple vertical tiers within a rack, and are installed as a sodium-sulfur battery package.
A sodium-sulfur battery module is a cell assembly contained inside a housing that is a heat-insulating module container. The cell has a structure in which molten metallic sodium serving as a negative-electrode active material is located in one part and molten sulfur serving as a positive-electrode active material is located in another part, and in which the both active materials are separated by a β-alumina solid electrolyte selectively permeable to sodium ions. The cell is heat controlled for each module at 300 to 350° C. Since the cell is a high-temperature cell which operates in such high temperature conditions, various safety measures have been taken.
However, there remains a possibility that the cell be broken to cause a high-temperature molten material to flow into the inside of the heat-insulating container and cross an inter-block sand layer to reach an adjacent block of the cells which short-circuit, and a large number of the cells be broken by heat generated by short-circuit current to cause fire to spread to the entire module. Moreover, there remains a possibility that, once a fire occurs, the fire be not confined to the inside of the casing which is hermetically sealed, and fire spread to horizontally and vertically adjacent modules within the package to cause the extensive fire.
Thus, as described in Patent Literature 1, it has been proposed to place a heat-resisting plate such as carbon cloth in the upper inside of a casing which included in a module, in order to prevent a flame from going outside at the time of fire. However, since the heat-resisting plate has a low strength, the heat-resisting plate easily deforms by pressure of a high-temperature molten material generated at the time of fire. As a result, there has remained a possibility that a top lid be broken by such pressure to cause the high-temperature molten material to spout and fire spread to an upper module. Furthermore, there has remained a possibility that the high-temperature molten material melt and penetrate the top lid by heat thereof to spout together with a combustion flame.